Non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, have a separator that electronically insulates the positive electrode from the negative electrode while holding an electrolyte. A porous film made of resin (i.e., porous resin film) is predominantly used as the separator, and polyolefin or other resin is used as a raw material of the porous resin film.
Such porous resin films deform at high temperatures. Therefore, under abnormal conditions where a battery becomes short-circuited and the temperature of the battery rises, the short-circuit may expand. Hence, it has been examined to form a porous heat-resistant layer that resists deformation even at high temperatures on an electrode. The porous heat-resistant layer is composed mainly of heat-resistant ceramic particles and therefore has a function of suppressing occurrence or expansion of short-circuits under abnormal conditions.
The ceramic particles of the porous heat-resistant layer are mainly aluminum oxide. Aluminum oxide does not contribute to battery reactions which occur in non-aqueous electrolyte secondary batteries. Also, aluminum oxide is highly heat-resistant and suited for controlling the pore size distribution of the porous heat-resistant layer.
Japanese Laid-Open Patent Publication No. 2003-142078 proposes using magnesium oxide as the ceramic particles. Magnesium oxide does not contribute to battery reactions occurring in non-aqueous electrolyte secondary batteries either, thus being stable.
The porous heat-resistant layer is formed on an electrode by using a slurry composed of a mixture of ceramic particles, a binder and a liquid component. After the slurry is applied onto the electrode, the liquid component needs to be removed by drying. However, the electrode has a current collector and an electrode mixture layer carried thereon, and the liquid component of the slurry permeates the electrode mixture layer before it is removed by drying. As a result, the thickness of the electrode mixture layer increases.
An increase in the thickness of the electrode mixture layer results in a decrease in the active material density. In consideration of the insertion of an electrode assembly composed of integrated positive and negative electrodes into a predetermined battery container (battery case), an increase in thickness needs to be considered in designing a battery. In this case, the energy density of the battery decreases. Also, even if a battery is carefully designed, it may be difficult to insert the electrode assembly into the battery container.
The final thickness of an electrode mixture layer may be limited by rolling the electrode mixture layer carried on a current collector with a strong pressure before forming a porous heat-resistant layer. However, excessive rolling causes the electrode mixture layer to separate from the current collector. Also, if an electrode with a porous heat-resistant layer is rolled, the porosity and pore size distribution of the porous heat-resistant layer are changed, so that the discharge characteristics are impeded. Further, since the ceramic particles of the porous heat-resistant layer damage the surfaces of reduction rolls, the life of manufacturing facilities is shortened.